Question

Find a1 for the arithmetic series with Sn = 77, an=30, and n-7

Answer 1

Just as in your other post,

Sn = n/2 (a1 + an)

Only this time we want a1

77 = 7/2 (a1 + 30)
154 = 7a1 + 210
-56 = 7a1
a1 = -8